Saturday, February 2, 2013

Cloud Decks of Gas Giant Planets

Figure 1: Artist’s impression of a gas
giant planet.

Brown dwarfs and gas giant planets such
as Jupiter and Saturn have hydrogen-helium dominated atmospheres that are very
different from the atmospheres of Earth-like planets. In the hydrogen-helium
dominated atmospheres of brown dwarfs and gas giant planets, a wide variety of
molecular species can condense to form clouds. This is expected to produce
multiple cloud layers in the atmosphere, where each cloud layer is made up of a
particular type of condensable species. By comparison, only water clouds
produced from the condensation of water exist in the atmospheres of Earth-like
planets.

Gas giant planets are known to exist
over a wide range of distances from their parent stars, from “star-hugging”
hot-Jupiters to isolated Jupiter-mass planets in the frigid depths of
interstellar space. Also, depending on its age, a gas giant planet is much
hotter when it is young and it will gradually cool over billions of years. As a
result, gas giant planets can have a wide range of atmospheric temperatures.
The atmospheric temperature of a gas giant planet strongly influences the
number of cloud layers it has and the position of its cloud layers. With
decreasing atmospheric temperatures, the more refractory cloud layers form at
progressively greater depths in the planet’s atmosphere and cloud layers
composed of more volatile condensable species become present at the upper
portions of the atmosphere.

When a condensable species forms a cloud
layer, the condensate is removed from the cooler overlying atmosphere and is no
longer available to react with other molecular species higher up in the
atmosphere. For example, the detection of hydrogen sulphide (H2S) in
Jupiter’s atmosphere by NASA’s Galileo entry probe indicates that iron (Fe)
must be sequestered into a cloud layer much deeper in the planet’s atmosphere
because the presence of Fe will lead to the formation of iron sulphide (FeS)
instead of H2S. In another example, the detection of monatomic
potassium (K) in the atmospheres of some brown dwarfs suggests that rock-forming
elements such as aluminium (Al) and silicon (Si) are removed from the
atmosphere due to cloud formation deeper down in the atmosphere. If Al and Si
were not removed, the potassium would have condensed into silicate minerals
such as orthoclase (KAlSi3O8) which would remove the
presence of monatomic potassium from the observable atmosphere of the brown
dwarf.

In the atmosphere of Jupiter, the
highest clouds are composed of a cirrus-like layer of ammonia (NH3) ice
crystals. Further down into the atmosphere are ammonium hydrosulphide (NH4SH)
and water (H2O) cloud layers. The troposphere of Jupiter’s
atmosphere is commonly defined from the 0.1 bar level (approximately 50 km
above 1 bar level) down to the 10 bar level (approximately 90 km below 1 bar
level). At the top of the troposphere, the temperature is about 110 K and at
the bottom of the troposphere, the temperature is about 340 K. The NH3
(0.6 to 0.9 bar), NH4SH (1 to 2 bar) and H2O (3 to 7 bar)
cloud layers are all located within the troposphere of Jupiter. Although
methane (CH4) also exists in the atmosphere of Jupiter, the
temperatures are too warm for it to condense to form clouds.

Deeper down into Jupiter’s atmosphere
are alkali, halide and sulphide cloud layers. These are then followed by silicate
and iron cloud layers at increasing atmospheric pressures and temperatures.
Finally, the deepest cloud layers consist of refractory species such as perovskite
(CaTiO3) and aluminium oxide (Al2O3) crystals.
In fact, in the atmospheres of the hottest gas giant planets, only cloud layers
consisting of refractory species exist in their atmospheres because the
temperatures are too high for other molecular species to condense. As a result,
the atmospheres of the hottest gas giant planets resemble the deepest regions
of Jupiter’s atmosphere.